Nanodiamond embedded polyaniline/polyvinylidene fluoride nanocomposites as microfiltration membranes for removal of industrial pollution

Membrane fouling remains a challenge to the membrane technology. Herein, we report the fabrication of composite membranes of polyaniline/polyvinylidene fluoride (PANI/PVDF) blended with nanodiamond (ND) with improved antifouling properties. The designed membranes were characterized by XRD, FTIR and SEM techniques. Characterization analysis revealed that addition of ND has maintained the structural integrity and porosity of composite membranes. The membrane permeation and antifouling performances were tested for hydrophilicity, porosity, pure water flux, shrinkage ratio, salt rejection of zinc acetate and copper acetate, and their fouling recovery ratio (FRR) measurements. A high solvent content ratio of 0.55 and a low shrinkage ratio of <12% due to enhanced hydrophilicity and porosity of the composite membrane with fouling-recovery of membranes to 88% were achieved. Separation of copper and zinc ions from aqueous solution was achieved. These findings imply that ND-based PANI/PVDF composite membranes can effectively serve as microfiltration membranes in industrial and municipal wastewater treatment.


Introduction
Rapidly increasing rates of the world's population and industrialization together with prevalent climatic changes, have aggravated water scarcity and contamination across the globe. 1,2larmingly, around one-h of the world's population is facing a severe water shortage in terms of limited access to clean and/ safe drinking water resulting in a signicant number of deaths every year. 1,3Thus, critical global water demand has challenged the scientic community to develop cost-effective and efficient technologies for water production and/or to recycle high-quality water.
Of all existing water treatment technologies such as adsorption, ion exchange mechanism, and precipitation reactions, 1,2 the membrane-based ltration technologies i.e., microltration (MF), ultraltration (UF), nanoltration (NF), and reverse osmosis (RO), have been acknowledged as the most cost-effective, environmentally friendly, and technologicallymatured. [4][5][6] Their widespread applications include, but are not limited to, desalination, brackish water soening, wastewater treatment, industrial water discharge decontamination, etc. [7][8][9] At present, polymeric membranes have received tremendous attention due to their unique characteristics including, interconnected pore structure, high surface area, exibility, and relatively low-cost processing. 8,105][16][17][18] The rationale behind membrane modication is to enhance the water permeance across the membrane maintaining a high solute-rejection rate without compromising the active membrane area.
Polyvinylidene uoride (PVDF) has been acknowledged as an excellent material in membrane sciences due to its incredible properties, such as remarkably high thermal and mechanical stability, chemical resistance, and exceptional membrane forming abilities. 19,20At the same time high hydrophobicity of PVDF compared to other polymer membranes makes it vulnerable to fouling which needs to be addressed.Numerous efforts have been devoted for antifouling of PVDF membranes via blending with hydrophilic nanoller/polymers, chemical oxidation, plasma treatment, etc.For instance, Choi et al. 21eported the modication of PVDF membrane via blending with poly(ethylene glycol) methyl ether methacrylate (POEM) gra co-polymer microltration membrane and investigated their antifouling properties.The graed membranes exhibited no irreversible fouling during ltration of different foulants i.e. bovine serum albumin, sodium alginate, and E. coli broth due to surface hydrophilicity of POEM polymer compared to the pristine PVDF membrane.Yoon et al. 22 investigated the surface modication of polyethersulfone electrospun ltration membrane by oxidation process using ammonium persulfate and noticed enhanced hydrophilicity and ltration ux of the modied membrane.Nasreen et al. 23 prepared electrospun nanobrous membranes via in situ polymerization of PVDF with hydroxyethylmethacrylate (HEMA) followed by coating with surface-charge chitosan polymer.The observed comparatively better ux and recovery ratio of PHEMA electrospun membranes due to HEMA's hydrophilic nature compared to PVDF membranes.The PVDF-based membranes mixed cellulose esters (MCE) and polyethersulfone (PES) have been used for activated sludge treatment by Fang et al. 24 Their ndings showed that pore-fouling was affected by the hydrophilicity, microstructure, and pore openings of the composite membrane.These results, among various other studies, 25,26 signify the concept of PVDF membrane modication via polymer blending, composite antifouling membranes, as a peculiar domain of membrane technology research.The incorporation of hydrophilic polymer into PVDF matrix noticeably enhances the composite membrane durability and ux rate by tuning the porosity of the membrane. 19,20,279][30][31] PANI-blended membranes have been reported to exhibit higher permeability and antifouling properties due to their hydrophobicity. 32,33Another strategy to enhance antifouling membrane properties is the incorporation of nanomaterials, nanollers, in the polymer composite membranes which signicantly affect the physiochemical characteristics of matrix material. 2,15,17Nanollers adhere to the polymer matrix via chemical bonding, increasing the polymer-ller phase compatibility which results in dramatic change of polymer blend behavior. 346][37] Recently, nanodiamonds (NDs), one of the most attractive allotropes of carbon, has emerged as a functional nanoller with outstanding properties such as high mechanical and thermal stability, homogenous size distribution, nontoxicity, and high surface area with tunable surface structures. 38,391][42] Previously we had reported the hydrophilicity of nanodiamonds in PDVF/ND composite microltration membranes. 43It was observed that a higher content level of ND (5%) has surprisingly enhanced the water ux, water content and porosity of the composite membrane.
Undoubtedly, polymer blending and nanollers' incorporation into the PVDF matrix is the novel strategy to achieve the desirable antifouling properties of hydrophobic PVDF membrane.Here we report the fabrication of poly(vinylidene uoride)-polyaniline (PVDF-PANI) nanocomposite micro-ltration membranes impregnated with nanodiamond (ND) llers (1-5 wt%) via solution casting method.We have presented the physical and chemical properties of PVDF-PANI polyblend and ND-incorporated PVDF-PANI nanocomposites micro-ltration membranes which propose these high-performance membranes may open up new avenues for engineering of nanofabricated membrane materials for wide range of applications.

Functionalization of nanodiamonds
Surface functionalization of nanodiamonds was performed according to the following procedure as reported earlier. 40In the rst step NDs were oxidized with the mixture of H 2 SO 4 and HNO 3 (3 : 1) at 90 °C for 1 hour under constant stirring.Subsequently the solution was ltered and washed repeatedly with deionized water until pH maintained to 7. In the second step, treated NDs were poured into a mixture of H 2 SO 4 and HNO 3 (9 : 1) and stirred for 3 days at 90 °C.The NDs obtained were again ltered and washed with deionized water until the pH reached to neutral.At the end, resulting material was treated with 0.1 M NaOH and 0.1 M HCl, respectively, followed by washing with deionized water.The obtained slurry was dried in vacuum oven for 24 hours at 100 °C to achieve surface functionalized NDs.

Fabrication of composite membranes
PVDF-PANI and PVDF-PANI/NDs composite blend membranes were fabricated prepared via solution casting method. 44Desired amounts of PANI and PVDF in ratio 2 : 1 (wt%) were added in the DMF solvent and sonicated for about half an hour at 60 °C for well dispersion.Aer complete dispersion, the solution was le still for 1 hour to remove the trapped air bubbles.The solution was then casted in the vacuum oven at 80 °C until a lm is obtained.

Membrane characterization
To ascertain the structural features of the composite membranes X-ray diffraction (XRD) analysis was conducted by Panalytical 3040/60 X ′ Pert PRO diffractometer in the range of 10°to 80°.FTIR analysis was performed over the scan range of 500-4000 cm −1 using 1000 PerkinElmer.The surface morphology was observed by Scanning electron microscopy (Quanta 600F).To evaluate the thermal stability and phase transformation of the composite membrane samples, thermal gravimetric analysis was employed using TGA/DA PerkinElmer USA in the temperature range 50-800 °C with scan rate was 10 °C min −1 .
2.5 Membrane permeation performances 2.5.1 Water ux study.A vacuum ltration setup was employed for estimation of pure water ux of the membranes, as the amount of water passing across the membrane per unit time per unit area under transmembrane pressure.The membrane was subjected to the pure water ux estimation at trans membrane pressure of 0.2 bar.Pure water ux was calculated under steady-state ow using following equation; 45 where J w is pure water ux (mL cm min −1 ), A is membrane area (cm), t is ltration time (min) and Q is amount of permeate during ltration time (mL).2.5.2Membrane porosity.The porosity of composite membranes was determined by measuring dry and wet membrane weight of pieces of membrane (1 cm dimension).In the next step, membrane was soaked in distilled water for 24 h and weighed out by mopping with blotting paper.Then wet membrane was dried at 70 °C in oven overnight and weighed again.From two membrane weights (wet and dry), the porosity of membrane were determined using formulae: 46

Pð%Þ
where P is porosity (g cm −2 ) of membrane, W 1 is wet membrane weight (g), W 2 is dry membrane weight (g) and A h is area of wet membrane (cm 2 ).2.5.3Solvent uptake measurements.For the solvent content/uptake estimation, the membrane was cut into four square pieces having dimension and an area of 1 cm and 1 cm 2 , respectively.These four different pieces of membrane were separately soaked in each solvent (water, methanol, ethanol, and propanol) for 24 h and weighed by mopping with blotting paper.The wet membrane was dried in oven at 70 °C overnight and the weight of dried membrane sample was measured.The equation for the calculation of water uptake is given as follow: where, W 1 and W 2 are the weights (g) of the wet and dry membrane, respectively.

Membrane shrinkage ratio (%).
For the shrinkage ratio estimation, a piece of wet membrane was taken and its length and breadth are determined.The piece was then dried at 100 °C overnight followed by re-measurements.From these values the shrinkage ratio was then calculated as: where a and b are the length and width of dry membrane and "a o " and "b o " are the length and width of wet membranes respectively.

Membrane antifouling performance (salt rejection and fouling recovery ratio)
The salt-rejection factor determines the amount of the salt retained by a membrane or in other words, it accounts for the capacity of a membrane to reject the undesired salt/compound from the feed mixture.On other hand, membrane fouling refers to the blocking of the pores in membrane due to the collection of solutes from the feed solution during ltration process.Hereaer, the fouling ratio accounts for antifouling ability of a membrane to fouling or % reduction in fouling.The salt rejection ratio of membranes was tested against heavy metals such as copper acetate and zinc acetate solutions (0.1 M each), model salts, under trans-membrane pressure of 0.2 bar.The concentration of salt was determined in the feed and ltrate was determined using conductivity meter as per following formula; where C p and C f are the salt concentration in ltrate (permeate) and salt concentration of feed, respectively.Membrane antifouling performance was tested by previously reported method. 47Aer measuring pure water ux (J w 1 ) of membrane at trans-membrane pressure of 0.2 bar, 0.1 M aqueous solution of each model salt (copper acetate and zinc acetate) was individually ltered through membranes for 30 min at same pressure.Aerward the membranes were ushed with water under identical conditions of time and pressure and pure water ux (J w 2 ) was measured.The antifouling recovery ratio (FRR) of membranes was then measured using following relation: Here J w 2 is the ux of cleaned membrane and J w 1 is the ux of pure membrane.

XRD of composite blend membranes
XRD patterns of the synthesized materials are shown in Fig. 1.
As shown in Fig. 1a, PANI with diffraction peaks at 20.05 and 40.02°was accorded to the pseudo-orthorhombic phase. 48The XRD pattern of pristine PVDF (Fig. 1b) with diffraction peaks 18.4, 20.18, 26.5, 33.91, 38.01, and 40.41°was corresponded to b-phase-PVDF with monoclinic structure. 49Fig. 1c demonstrated the diffraction peaks of functionalized ND at 32.1, 43.2 and 75.0°corresponding to sp 3 hybridized carbon structure. 43imilar observations have been reported in the literature. 43,48,49he XRD prole of PANI/PVDF composite membranes doped with varying concentration of NDs from 1-5% (Fig. 1d-h) illustrates broad diffraction peaks between 20°-40°due to perpendicular and parallel periodicities of the PANI polymer chain.Upon incorporation of NDs in these PANI/PVDF composite membranes, the characteristic peaks of NDs are observed at 20.5 and 43.01°while a broad peak is also observed at 38.72°indicative of b-phase of PVDF.These observations indicate that addition of ND has maintained the structural integrity of the polymer blend.Moreover, with the increase in the concentration of NDs increase in area under the diffraction peaks and intensity are observed.This could be attributed to the reason that NDs have acted as lling agent in these composites resulting in high mechanical strength and formation of defects. 40

Scanning electron microscopic (SEM) of composite microltration membranes
Scanning electron microscopic (SEM) micrographs of undoped PVDF/PANI membrane, 1 wt% nanodiamonds (1ND-PANI), 3 wt% nanodiamonds (3ND-PANI) and (5ND-PANI) are shown in Fig. 3.The SEM image of composite membrane with 5 wt% of ND is porous as compared to other membranes in series.The porosity as well as pore size has increased upon the successive inclusion of nanodiamonds depicting the role of nanoller as a pore forming agent.Moreover, the surface is found to be homogenous and no segregates of nanodiamonds are observed which proves an efficient interaction of the matrix (PANI/PVDF) with the ller (NDs) material. 52This establishes the fact that NDs are of smaller size and are well embedded in the polyblend matrix whereas; an increase in the pore size is attributed to repulsions between the polymeric chains and/or matrix and ller materials.It has been reported that electrostatic repulsions between polymer chains hinder their coagulation resulting in the formation of wide pores.The addition of ND might have caused the pattering of the pores. 43

Thermal gravimetric analysis (TGA) of composite microltration membranes
The TGA prole of undoped PVDF-PANI membrane and its composites with nanodiamonds is displayed in Fig. 4.These results clearly illustrate the increase in the thermal stability of the composite membranes with increasing nanodiamond content.The nanodiamonds owing to its signicant thermal properties contribute thermal stability to the composite membranes.The TGA prole represents thermal decomposition as a one step process which is due to the evaporation of adsorbed gases, moisture and organic impurities which might be incorporated in the matrix during the synthesis process.
The temperature prole of percent weight losses (5%, 10%, and maximum) of ND-doped PANI/PVDF composite membranes measured during TGA analysis are presented in Table 1.
5% weight loss appears in the temperature range of 406-501 °C, 10% weight loss takes place in temperature range of 440-512 °C, and maximum weight loss occurs between 540-556 °C whereas the residual amount of the compound varies from 48-65% for undoped PANI/PVDF and its ND-based composite membranes.The shi in weight loss and increase in residue value can be explained on the basis of an increase in interfacial interactions between the ller and the matrix materials with the increase in concentration of nanodiamonds.3.5 Properties of ND-based PANI/PVDF composite membranes 3.5.1 Porosity and shrinkage ratio.Fig. 5 illustrates the membrane porosity measurements.It is evident from Fig. 5 that there is continuous increase in the porosity of the membranes as the concentration of NDs is increased from 1% to 5%.The increase in porosity from 2.41g cm −2 to 5.43g cm −2 by the addition of NDs in polyblend is attributed to the increasing hydrophilicity of the membranes as NDs are functionalized with hydroxyl (-OH) and carboxyl (-COOH) groups. 38,41Hence, the increase in hydrophilicity upon incorporation of NDs in the presence of PANI signicantly contributes to the increase in composite membrane porosity.In our previous studies on NDbased PVDF membranes, The membrane shrinkage ratio measurements are illustrated in Fig. 5 which shows that the % shrinkage ratio is decreased from 19% to 12% as the concentration of ller is increased from 1% to 5%.This is because the porosity of membranes is inversely related to its shrinkage ratio.The shrinkage ratio of ND-based PANI/PVDF composite membranes is found to be less than 20% in current study, one of the major requirement of wastewater treatment, implying effectives of investigated membranes in wastewater applications. 53.5.2Solvent contents.In order to determine the membrane selectivity four different solvents were selected as water, methanol, ethanol, and propanol having dielectric constant (3) 80.4, 33.1, 24.3, and 20.1, respectively.The results of solvent uptake/content of ND-based PANI/PVDF composite membrane is presented in Table 2.It is evident from Table 2 that the water content of the membranes has increased from 0.361 to 0.55 with increasing concentration of NDs from 1% to 5%, respectively.This is attributed to the enhancement of membrane porosity and hydrophilicity induced by addition of PANI and nanollers which proves that incorporation of NDs in polyblend has signicantly enhanced the permeability of composite membranes.Moreover, solvent content of these membranes has decreased from more polar solvent (water) to less polar solvents (propanol) depending upon their polarity.From results of solvent content, it is evident that solvent content of 5% ND composite membrane has maximum value for water and least for propanol, on other hand it has maximum value compared to 1% ND substitution level.The results indicate that incorporation of ND in PANI/PVDF polyblend has enhanced the hydrophobicity and porosity of the composite membranes.

Antifouling properties of ND-based PANI/PVDF composite membranes
Pure water ux, rate of membrane ltration per square foot of its surface area, is the foremost criteria determining the fate of membrane for utilizing in the waste water treatment.As higher ux rate implies less membrane surface area requirement which in turn lowers the installation cost of treatment unit. 54In other words, the membrane permeability, ux rate per 1psi trans-membrane pressure, is the desirable feature of good quality membranes.Installation of the membrane in the treatment plant-assembly requires higher value of water ux at possibly low trans-membrane pressure that can satisfy the requirement.The water ux of ND-based PANI/PVDF composite membranes (mL cm −2 min −1 ) measured at 0.2 bar pressure is given in Table 3.As the concentration of NDs is increased from 1% to 5% the pure water ux increases from 8.0 mL cm −2 min −1 to 15.5 mL cm −2 min −1 due to enhancement of composite membrane porosity and hydrophilicity caused by NDs incorporation in the presence of PANI.
The salt rejection (SR) and fouling recovery ratio (FRR) of NDbased PANI/PVDF composite membranes were tested against two heavy metals including copper acetate and zinc acetate solutions (0.1 M each), as illustrated in Fig. 6.As the concentration of NDs in polyblend is increased from 1% to 5% the % salt rejection increases from 35% to 77% and 36% to 90% for copper acetate and zinc acetate, respectively, which indicates  Paper RSC Advances that addition of ND has effectively enhanced the adsorptive capacity of membrane resulting in higher salt rejection.Besides, FRR increases from 75% to 88% and 75% to 90%, as shown in Table 3, with increasing ND concentration from 1-5wt% for copper acetate and zinc acetate salts, respectively.These results established the fact that both the salt rejection and fouling recovery ratio of composite membranes are boosted up by increasing the content of ND ller.The membrane with higher loading of ler (5 wt% of NDs) showed the best results compared to 1% ND-PANI membrane.

Conclusion
A series of nanocomposite microltration membranes were fabricated by incorporating varying amounts of nanodiamond llers (1-5% wt%) into blended polyaniline-polyvinylidene uoride (PANI/PVDF) backbone via solution casting approach.Thermal analysis exhibits one-step decomposition of composite membranes with a maximum weight loss in the temperature range of 540-556 °C.XRD analysis exhibited the characteristic diffraction peaks at 20.5°, 36.71°, and 43.61°characteristics for ND-doped PANI/PVDF nanocomposite membrane exhibiting phase a / b transition of PVDF caused by ND inclusion.Membrane with higher loadings of ller (NDs) exhibited maximum porosity, water content, pure water ux, salt rejection and fouling recovery ratio.Thus, doping the PANI/PVDF membranes with NDs increases the efficiency of water ltration membranes.Furthermore, it is established that these membranes belong to the category of microltration as the porosity of the membranes corresponds to microltration membranes processes.In addition, the % shrinkage ratio was found less than 20% in all the cases which agrees with the recommended ratio for effective ltration to be less than 20%.These results indicate that PANI/PVDF composite micro-ltration membranes impregnated with ND llers can effectively be used in wastewater treatment.

Fig. 2a illustrates
Fig. 2a illustrates the FTIR spectrum of undoped PANI/PVDF polyblend.The absorption band appearing in the region of 825-870 cm −1 corresponds to C-F stretching vibration of PVDF, whereas the bands around 1082 and 1155 cm −1 represent in-plane bending vibrations of aromatic C-H group and C-C band vibrations, respectively.The characteristic band of C-N + stretching vibrations of PANI appears at around 1409 cm −1 and C-H vibration of CH 2 group is represented by appearance of absorption band at around 2366 cm −1 . 50Fig.2b-g presents FTIR spectra of PANI/PVDF polyblend with varying amounts of ND (1-5 wt%) which resemble the absorption pattern of the polymer backbone.Besides, appearance of new absorption bands around 1600 cm −1 and ∼3700 cm −1 corresponding to C]O of COOH group and OH group, respectively conrm the inclusion of ND in PANI/PVDF polyblend.These ndings are consistent with previously reported literature.42,43,51

Table 1
Temperature profile of percent weight loss of undoped PANI/ PVDF and NDs-based PANI/PVDF composite membranes

Table 2
Solvent Contents of NDs-PANI/PVDF Composite membranes